A typical compressor performance map is a graphical representation of percent of rated head plotted with efficiency contours against percent of rated volumetric flow at various speeds. The point on the performance map at which the compressor operates with maximum efficiency is the design point. The selection or rating point is the point on the performance map at which the compressor is being applied to a particular system. It is not unusual for refrigeration compressors to be selected for heads and capacities above their peak efficiency regions. The maximum flow rates approach a limiting value determined by the relative velocity of the refrigerant entering the impeller since, as this velocity approaches a sonic value, the flow becomes choked and further increases become impossible. In a compressor equipped with pipe or vaned diffuser downstream of the impeller, maximum flow is determined by choked flow in the diffuser passage.
Under partial load conditions, satisfactory compressor operation is limited by an instability known as surge in which the refrigerant alternately surges backward and forward through the compressor accompanied by noise, vibration and heat. Surging can be distinguished from other kinds of noise and vibration by the fact that its flow reversals alternately unload and load the driver. Motor current varies markedly during surging, and turbines alternately speed up and slow down.
Another kind of instability that occurs in centrifugal compressors is incipient surge or stall which may occur near, but at lighter loading than, the true surge envelope. This phenomenon involves the formation of rotating stall pockets or cells in the diffuser. Rotating stall is an essentially two dimensional instability in which regions of reduced or reversed mass flow rotate around the compressor annulus. Typically, the frequency of rotating stall is much higher than the frequency of surge. A roaring noise is produced at a frequency determined by the number of cells formed and the impeller running speed. The driver load is steady but excess vibration results from rotating stall.
U.S. Pat. No. 5,683,223 discloses surge detection apparatus employing multiple sensors. More specifically, pressure is measured in the diffuser on the suction and pressure sides. Differences between pressures measured by different sensors is determined and these signals along with others representing other parameters are used to detect surge. In order to protect industrial compressors from operating in rotating stall, non-contact proximity probes may be used to measure the vibration. The major disadvantages are cost and compatibility of the transducer with refrigerants.